1. Field of the Invention
This invention relates to sunscreens that absorb specific wavelengths that we have found induce destructive enzymes in the skin, and to the use of these specific wavelengths in treating skin conditions.
2. The State of the Art
With regard to photodamage to skin from the sun, the prevailing view is now that both UVB and UVA radiation should be blocked to prevent damage to the skin. It has been known for some time that UVB, while enabling the skin to produce Vit. D3, nevertheless also produces erythema (sunburn). If the UVB radiation reaches a threshold dose level termed the minimum erythemal dose (MED), then sufficient UVB radiation has been delivered to the skin to cause visible erythema. UVA radiation is orders of magnitude less erythmogenic than UVB radiation, but is nevertheless damaging to the skin. The art generally considers the damaging regions to be 280-320 nm for UVB, and 320-360 nm for UVA. UVB sunscreens are typically evaluated by their ability to prevent erythema, and that is how the Sun Protection Factor (SPF) is typically defined. Although less erythmogenic, UVA sunscreens are often tested in the same manner, or analogously to determine whether the compound screens against induction of pigment in the skin upon UVA exposure. See generally, Sunscreens: Development, Evaluation, and Regulatory Aspects, ed. by N. J. Lowe et al. (New York: Marcel Dekker, Inc., 1997), the disclosure of which is incorporated herein by reference.
Present sunscreen formulations now include a mixture of separate compounds for absorbing UVA and UVB radiation. Commercially approved preparations include a UVB blocker, such as a p-methoxycinnamate or an aminobenzoate, and a UVA blocker, such as a benzone or an anthranilate. These compounds generally absorb the incoming UV photon and reradiate a lower energy photon. While typically less cosmetically desirable, physical blockers, such as zinc oxide, generally provide better protection, at least in part because most people do not apply a sufficient amount of sunscreen, or apply it unevenly. In theory, an amount of 2 mg/cm2 of sunscreen per skin area is to be applied to maintain the sun protection factor (SPF) value, although the amount typically applied in practice by individuals in recreational settings is much less.
The ideal compound would be one that absorbs well over the entire UVA/B spectrum, but no such compound has been identified to date. To further complicate matters, the solvent or medium in which the absorber is formulated will affect its absorbtion spectrum. While these shifts in absorbtion spectra can be qualitatively estimated, to some extent sunscreen formulation is an empirical art.
On the other hand, dermatologists have been resorting to the use of UVA and UVB radiation for the treatment of various skin conditions. UVB has been used for treating acne for some time. More recently, UVA radiation has been used for treating various dermatological fibrotic conditions: M. C. Polderman et al., “Ultraviolet A-I phototherapy for skin diseases,” Ned Tijdschr Geneeskd, 1999 May 1, 143(18):931-4; M. El-Mofty et al., “Low-dose broad-band UVA in morphea using a new method for evaluation,” Photodermatol Photoimmunol Photomed 2000 Apr. 16(2):43-9; J. Dutz, “Treatment options for localized scleroderma,” Skin Therapy Lett 2000, 5(2):3-5; J. W. Steger et al., “UVA therapy for scleroderma,” J. Am. Acad. Dermatol., May 1999, part 1, Vol. 40, No. 5.